[AbstractCallSite] Check that callback callee index is within call arguments

Summary:
AbstractCallSite::getCallbackUses() does not check that callback callee index from
the callback metadata does not exceed the total number of call arguments. This patch
add such validation check.

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: hiraditya, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78112